(1) Field of the Invention
This invention relates to an optical disk used for data recording and reproduction, especially to a multi-layered optical disk having multiple recording layers.
(2) Description of the Prior Art
In recent years, this type of optical disks have been developed actively due to the large memory capacity and high access speed. An optical disk shown in FIG. 1 has been proposed in order to further increase the memory capacity.
This optical disk 12 comprises three recording layers 8a through 8c formed of a photochromic material such as spyropyrene, the layers being interposed between a pair of bases 13. The recording layers 8a through 8c have sensitivity peaks in wavelengths λ1 through λ3 (FIG. 2), respectively while allowing lights having the other wavelengths to transmit therethrough.
Data recording and reproduction is done in the following way. A light is emitted from a light source 9, such as a laser, which varies wavelengths, and focused into an extra fine light beam by a focusing optical system 10, thereafter the light is illuminated on the disk 12. The light is transmitted through the recording layers 8a, 8b and 8c and is detected by a light detector 11 provided on the other side from the light source 9.
Data recording will be described in more detail. If the light emitted from the light source 9 and illuminated on the disk 12 has a wavelength λ2, it is transmitted through the recording layers 8a and 8c but is absorbed into the recording layer 8b, whereby a data is recorded in the layer 8b. 
For data reproduction, only the data recorded in the layer 8b can be retrieved by illuminating a light of λ2.
As apparent from the above, memory capacity is increased by providing more recording layers.
However, providing more recording layers enlarges the total thickness of the recording layers. In order to record and reproduce data in such a thick disk only by use of wavelength difference without detecting exact positions of the layers, the light beam should have quite a large diameter, which prevents high density recording.
Also, the large light beam diameter causes crosstalks between neighboring tracks.